A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
It has been proposed to immerse the substrate in the lithographic projection apparatus in a liquid having a relatively high refractive index. e.g. water, so as to fill a space between the final element of the projection system and the substrate. However, another fluid may be suitable, particularly a wetting fluid, an incompressible fluid and/or a fluid with higher refractive index than air, desirably a higher refractive index than water. Fluids excluding gases are particularly desirable. The point of this is to enable imaging of smaller features since the exposure radiation will have a shorter wavelength in the liquid. (The effect of the liquid may also be regarded as increasing the effective numerical aperture (NA) of the system and also increasing the depth of focus.) Other immersion liquids have been proposed, including water with solid particles (e.g. quartz) suspended therein, or a liquid with a nano-particle suspension (e.g. particles with a maximum dimension of up to 10 nm). The suspended particles may or may not have a similar or the same refractive index as the liquid in which they are suspended. Other liquids which may be suitable include a hydrocarbon, such as an aromatic, a fluorohydrocarbon, and/or an aqueous solution.
In the lithographic apparatus, use is made of a movable support to hold and position an exchangeable object such as the substrate or the patterning device. In a scanning type lithographic apparatus, a movable support is used to support the substrate in order to make the scanning movement. The patterning device may also be supported on a movable support. The movable support is able to position the substrate or patterning device with high accuracy.
To obtain a high accuracy, a known movable support is assembled from a long stroke part, movable with respect to a reference object such as a frame, and a short stroke part, movably arranged with respect to the long stroke part. The short stroke part is configured to support the exchangeable object. The maximum stroke of the long stroke part with respect to reference object is relatively large, while the stroke of the short stroke part with respect to the long stroke part is relatively small.
A long stroke actuator is provided to actuate the long stroke part with respect to the reference object. A short stroke actuator is provided to actuate the short stroke part with respect to the long stroke part. Such long stroke actuator is for instance a linear motor, and may not be very accurate. The main task of the long stroke actuator is to keep the stator part of the short-stroke actuator in the vicinity of the moving part. The short stroke actuator is designed to position the short stroke part with high accuracy.
In order to control the position of the exchangeable object, the position of the second support system for supporting the substrate is determined by a position measurement system, for instance an interferometer system or an encoder system. This measurement is for instance performed in three planar degrees of freedom or in six degrees of freedom. The measured position is compared with a desired position. The position error, i.e. the difference between measured and desired position is fed into a controller which on the basis of this signal provides a control signal which is used to actuate the short stroke actuator.
The long stroke actuator is controlled by using a signal based on the difference between the actual position of the short stroke part and the long stroke part as an input signal for the long stroke actuator controller. The output of this controller makes the long stroke part follow the movements of the short stroke part, therewith keeping the desired position of the short stroke part within the range of the short stroke actuator.
The short stroke actuator may be of the Lorentz type to enable isolation from long-stroke vibrations. Such Lorentz type actuator has a small stiffness. Any other type of actuator having a small stiffness and high accuracy may also be used accurately to control the position of the exchangeable object support by the movable support. The input of a Lorentz actuator is an electrical current, substantially proportional to the desired force.
The force-type-actuator in the stages may limit the achievable feed-forward effect from one stage to the other (e.g. substrate table error fed to patterning device support). In this feed-forward, the position error of one stage needs to be differentiated twice to generate a feed-forward force, which costs one sample delay. This leads to a delayed response of the other stage, limiting positioning accuracy of the stages relative to each other. This feed-forward performance is further limited by calculation delay, amplifier (DAC) delay, and higher-order dynamics of the short-stroke system.